Uses copy request to flatten the layers to do screen rotation animation.

ash/rotator/screen_rotation_animator.h

Index: ash/rotator/screen_rotation_animator.h
diff --git a/ash/rotator/screen_rotation_animator.h b/ash/rotator/screen_rotation_animator.h
index b541106e88030c68a5217cc03451e46268688eae..b68b45ee41b3049d5f3f11143272dd83ee82b82b 100644
--- a/ash/rotator/screen_rotation_animator.h
+++ b/ash/rotator/screen_rotation_animator.h
@@ -13,6 +13,10 @@
 #include "base/observer_list.h"
 #include "ui/display/display.h"
 
+namespace cc {
+class CopyOutputResult;
+}  // namespace cc
+
 namespace ui {
 class AnimationMetricsReporter;
 class Layer;
@@ -59,7 +63,23 @@ class ASH_EXPORT ScreenRotationAnimator {
   friend class ash::test::ScreenRotationAnimatorTestApi;
   struct ScreenRotationRequest;
 
-  // Set the screen orientation to |new_rotation| and animate the change. The
+  void StartRotationAnimation(
+      std::unique_ptr<ScreenRotationRequest> rotation_request);
+
+  void RequestCopyRootLayerAndAnimateRotation(

[bruthig, 2017/03/30 18:28:24]

Some docs on these functions would be helpful.  Especially mentioning the ones
that are asynchronous.

[wutao, 2017/03/30 21:25:41]

Done.

+      std::unique_ptr<ScreenRotationRequest> rotation_request);
+
+  void OnRootLayerCopiedBeforeRotation(
+      std::unique_ptr<ScreenRotationRequest> rotation_request,
+      std::unique_ptr<cc::CopyOutputResult> result);
+
+  // Recreates all |root_window| layers.
+  void CreateOldLayerTree();
+
+  // Requests a copy of |root_window| root layer output.
+  void CopyOldLayerTree(std::unique_ptr<cc::CopyOutputResult> result);
+
+  // Sets the screen orientation to |new_rotation| and animate the change. The
   // animation will rotate the initial orientation's layer towards the new
   // orientation through |rotation_degrees| while fading out, and the new
   // orientation's layer will be rotated in to the |new_orientation| through